β-Agostic Interactions in 15-Valence-Electron 9-BBN Hydroborate Half-Sandwich Titanium(III) Complexes

A series of new mono-Cp- or Cp*-based titanium(III) 9-BBN hydroborate derivatives with either 15- or 17-valence-electron configurations were synthesized, and their structures were determined by single-crystal X-ray diffraction analysis. These studies revealed that the 15-valence-electron complexes [Cp*Ti(III)Cl{(μ-H)2BC8H14}(THF)] (2) and [Cp*Ti(III){(μ-H)2BC8H14}2] (6) show the presence of β-agostic interactions between the Ti(III) metal center and the α-hydrogens on the 9-BBN ligand in the solid state, while the 17-valence-electron complexes [CpTi(III){(μ-H)2BC8H14}2(THF)] (3), [Cp*Ti(III){(μ-H)2BC8H14}2(THF)] (4), and [Cp*Ti(III){(μ-H)2BC8H14}2(NC5H5)] (7) display no agostic interactions. Agostic interactions in Ti(III) species are uncommon. While the Ti···H distance can be indicative of an agostic interaction, other structural parameters provide more reliable indication of such an interaction. These are the shortening of the Ti−B distance, the lengthening of the B−C(α) distance, the sharpening of the Ti−B−C(α) angle, and the narrowing of the dihedral angle between the TiH2 and H2B planes in 9-BBN ligands that adopt an agostic interaction with titanium.